This invention relates to an anchor. More particularly, this invention concerns an anchor assembly or rockbolt for mouting in a bore hole drilled in rock.
In mining and like operations it is frequently necessary to have a secure anchor to which may be attached snake-lines, guy-lines and the like. To this end a hole is bored in the rock and an anchor assembly is mounted in this hole.
In the commonest type of anchor or rockbolt, a rod is provided at one end with a foot piece that can either be secured adhesively in the base of the bore hole or which can be spread by rotation of the rod into tight engagement with the inner wall of the bore hole. At its outer end the rod is fitted with an anchor plate or washer which is locked against the rock wall face over the mouth of the bore hole by means of a nut. Thus, the rod is pre-stressed between its two ends and is therefore firmly secured in the bore hole.
A simpler arrangement simply uses the adhesive setting or mounting of an eye bolt or the like in the bore hole. Such a system is considerably simpler, but has the disadvantage that it yields readily to rock loosenings. In addition, since the entire stress on the anchor is taken up at the point where the neck of the rod extends out of the adhesive mass in which it is set, the anchor can readily break at this point when stressed beyond its elastic limit.
Since such anchors or rockbolts are necessary for supporting and holding in place many structures on which mine safety depends it is necessary that they be as secure as possible. In certain types of rock which have limited holding power and cohesiveness it is impossible to avoid at least limited shifting of the rock. Such a shifting is normally effective radially on the anchors extending into the mine. The above-described anchors or rockbolts are unable to withstand such stresses. Thus, they often rip loose from the rock, or break when so stressed. It is impossible to ascertain whether a particular anchor is ready to break. Thus, if a slight shifting in the rock has been detected it is often necessary to pull anchors and replace them or to place new anchors, as even limited shifting often stresses the prior-art anchors or rockbolts so that even a very slight shift thereafter can cause them to break.